Forbes is serving a life sentence in prison after he admitted to killing Monge in Denver and admitting to the attempted murder of Lydia Tillman in Fort Collins. Forbes pleaded guilty to attacking Tillman and then setting her apartment on fire.

Forbes also wrote that Tillman "is the happy ending because without her, I never would have admitted to killing Kenia or leading cops to where I buried her."

Forbes wrote he didn't know Monge was "only" 19 years old.

"If I had known, I wouldn't have taken advantage of her," he wrote.

"A classic psychopath," said Dr. Kim Gorgens, a clinical associate professor and director of continuing education at the University of Denver's graduate school of professional psychology.

Gorgens has not evaluated Forbes herself, but one of her classes, Psychology of Criminal Behavior, wrote him as well and received a 60-plus page response. In some ways, it is similar to the one Bolton received.

"It certainly plays to contemporary theories about remorse for anyone who wants to believe that people are good that you can repent for sins: I regret it now, I'm a better person and I appreciate it," she said about the letter to her class. "These are folks who characteristically lack empathy. They just don't have a conscience. They prey on victims in society in a way that is so foreign to the rest of us who think, 'God, how could you do that?'"

Gorgens says her class dissects criminals' histories, backgrounds to figure out why people do horrible things. In the 11 years she's taught this class, students have studies six criminals a year, often high-profile ones.

"Ultimately, you hope something could be fruitful in acting more quickly to intervene, say you're working with juvenile offenders, to recognize risk immediately and be able to respond more quickly," she said.